FIG. 1 illustrates in section a conventional fluid control solenoid valve, which comprises a substantially cylindrical housing 1 made of a magnetic material. The housing 1 defines therein a fluid passage 2 including an inlet port 3 formed in the bottom wall 4 of the housing 1 and outlet ports 5 formed in a cylindrical side wall 6 of the housing 1 at equal circumferential intervals. The housing 1 also comprises a magnetic core 7 having a drain port 8 therein and a flange portion 9 at which the core 7 is secured to the side wall 6 of the housing 1.
A valve seat 10 having a valve port 11 for allowing the passage of the fluid is disposed in the fluid passage 2. In order to open and close the valve port 11, a cylindrical plunger 12 having a substantially frustoconical valve body 13 at its one end is axially slidably disposed within the housing 1 so as to be movable between a close position shown in FIG. 1 in which the valve body 13 engages the valve seat 10 to close the valve port 11 and an open position in which the valve port 13 disengages from the valve seat 10 to open the valve port 11. The plunger 12 has a cylindrical sliding contact surface 14.
The housing 1 is provided therein with a substantially ring-shaped plunger guide 15 having an inner circumferential surface or a guide surface 16 disposed around the cylindrical sliding contact surface 14 of the plunger 12. The sliding contact surface 14 and the guide surface 16 together provide an annular sliding contact region 17 defined between the sliding contact surface 14 of the plunger 12 and the gudie surface 16 of the plunger guide 14 for allowing a guided axial movement of the plunger 12 between the open position and the closed position.
The fluid control solenoid valve also comprises a compression coil spring 18 disposed within a large-diameter portion 19 of the drain port 8. The spring 18 is disposed between a shoulder portion 20 of the drain port 8 and an upper end 21 of the plunger 12 for biasing the plunger 12 toward the closed position shown in FIG. 1.
In order to magnetically drive the plunger 12 from the closed position to the open position against the spring action of the coil spring 18, a solenoid coil 22 is mounted within the housing 1. The solenoid coil 22 is wound on an electrically insulating bobbin 23 which is mounted on an axially extending portion of the magnetic core 7 to bridge between the core 7 and the plunger 12. The solenoid coil 22 wound on the bobbin 23 is fixedly supported by the flange portion 9 of the core 7 and the side wall 6 of the housing 1 and the plunger guide 15. A lead conductor 24 is provided to supply electric power to the solenoid coil 22. The solenoid coil 22, when energized, generates a magnetic flux extending through the magnetic core 7, the plunger 12, the plunger guide member 15, the housing side wall 6 and back to the magnetic core 7, thus magnetically driving the plunger 12 from the closed position against the biasing action of the coil spring 18.
When the fluid control solenoid valve is used in a hydraulic circuit such as a hydraulic circuit for shifting an automotive transmission, hydraulic fluid is filled in the fluid passage 2. When the solenoid coil 22 is not energized, the compression spring 18 urges the plunger 12 and the valve body 13 against the valve port 11 of the valve seat 10 to close it. Then, the fluid passage 2 is closed so that no fluid flow occurs between the inlet and the outlet ports 3 and 5.
When the solenoid coil 22 is energized, the magnetic flux generated by the solenoid coil 22 passes through the above-mentioned magnetic circuit including the plunger 12 to attract the plunger 12 to axially slide along the guide surface 16 of the plunger guide member 14 until it abuts against the end surface of the core 7. This causes the valve body 13 to separate from the valve seat 10 to open the valve port 11, allowing fluid communication between the inlet port 3 and the outlet port 5 through the fluid passage 2. During this time, the drain port 8 is also communicated to the inlet port 3 through an annular narrow clearance or a sliding contact region 17 defined between the sliding contact surface 14 of the plunger 12 and the plunger guide surface 16 of the plunger guide 15.
In certain hydraulic circuits, such as those used with automotive transmission mechanisms, the hydraulic fluid often entrains solid particulate foreign matter such as dust or metallic particles. The foreign matter entrained in the hydraulic fluid often enters into the sliding contact region 17 between the plunger 12 and the plunger guide 15 and the foreign matter bites into and is caught between the sliding surfaces 14 and 16. Therefore, the smooth sliding movement of the plunger 12 is impeded and the plunger 12 often sticks to the plunger guide 15. Fluid leaks may even occur in the valve.